Abstract

Wet-chemical analyses (41) of tourmaline from granitic pegmatites (barren, barren-pocket, beryl, lepidolite types) in the Moldanubicum, Czech Republic revealed that members of the oxy-subgroup - common oxy-schorl, minor oxy-dravite and rare oxy-foitite are more abundant relative to the relevant members of the hydroxy-subgroup. The primary substitution mechanisms in tourmaline show combination of heterovalent substitutions: YAlWO YR2+−1W(OH)−1, X□YAl2WO XNa−1YR2+−2W(OH)−1, X□YAl XNa−1YR2+−1 and X□W(OH) XNa−1WO−1, and homovalent substitutions: Fe2+Mg−1 and (OH)F−1. Tourmalines with the chemistry expressed by the general formula X(Na0.5□0.5)Y(R2+2Al)ZAl6(BO3)3Si6O18V(OH)3W(O0.5OH0.5) crystallized in very similar PT conditions in granitic systems saturated on Na, Al, Si and H2O, it indicates the importance of short-range order requirements on tourmaline chemical composition. Abundance of heterovalent substitutions involving the W-site requires determination of light elements (H, B, F, Li) and Fe2+/Fe3+ in tourmalines to specify substitution mechanisms with certainty. Normalization of electron-microprobe data of (Fe, Mg)-rich, (Ca, Li, F)-poor tourmalines from granitic pegmatites on (OH, F)3.5O0.5, which is more probable than (OH, F)4, seems to be suitable.